It is Teacher Appreciation Day and Billy decides to be nice to Mrs. Butterbean. He gives her an apple which is infested with a lot of nasty creatures. She eats the apple without washing it and she is about to die. Grim then appears to take Miss Butterbean to the Underworld, but Mandy says that she didn't die.
Principal Goodvibes appears and says that their needs a substitute. He sees Grim and asks him to be the substitute. Grim is reluctant to be the teacher, until Principal Goodvibes says that he will also get Miss Butterbean's starting salary at $2.00/hour. Grim then immediately accepts the position as the teacher.
The next day, Grim comes to and Sperg tries to pull a prank on Grim. Grim then takes his scythe and transports Sperg to the Detention Dimension forever. Grim continues to do this everyday. He does this until it is only Mandy left in the oom. Grim tricks Mandy into drawing a portal on the board that will trap her in the Detention Dimension. Finally, Mandy says that she will get revenge on Grim for trying to get them out of his life forever.
Mandy goes to the Detention Dimension to see everybody getting terrorized. Billy is getting punished by cleaning erasers, but Mandy suggests otherwise. Miss Slither then says that she hates Mandy. Grim appears and Mandy tricks Grim, so that he gives her his scythe. She returns to the real world vowing to saving Billy in some other episode.